As a guy who has had a drivers license since 1964 and one who actually drove a 64 Mustang (64 and a half they called it) and owned several cars from that era, heres my take.
On a cold start, push the gas pedal down 3/4 of the way and let up. It should start right away. If it starts and sputters or puts out black smoke, you need a new pull off valve. It is vacuum operated, has a rubber diaphragm which is prone to rot every decade or so and is not part of a rebuild kit. They don’t cost much and are easy to replace.
On a hot start, do not touch the gas, just turn the key, it should start right up. IF you flood the engine, which is easy to do on a hot start, then with the key off, push the gas pedal to the floor and hold it there for thirty seconds. After the thirty seconds, keep you foot on the floor and turn the key. The first crank generally clears out the flooding, it will start on the second cranking sound. It will start and rev up very quickly so get that foot off the gas the second it starts to prevent damage to the engine.
Now for the brakes. The brakes were very adequate for 1965. The reason that cars of that era didn’t stop fast was the tires, not the brakes. You could easily lock up any street tires from that era. If you put a booster on drum brakes, it makes them very touchy, especially on a light car like the Mustang.
Two things today make the brakes feel less adequate. The tire are a lot better and don’t lock up so quickly. You can add a lot more pressure to the brakes before reaching lockup so it feels like they don’t work as well.
Second is that brakes do not use asbestos anymore. The new friction materials do not work as well so more pressure is needed. Couple that with the fact the the best friction materials, the ceramics and carbon fiber friction materials aren’t available for brake shoes. If you can find sintered metallic shoes, they work better, but they do need a little more pressure when cold.
IIRC, the early Mustangs also had a single cylinder master cylinder. They work just fine, but they do not have a backup like a dual master cylinder does. If you stay all original, just be sure to meticlously maintain those brakes. If you find a good trustworthy mechanic, have him/her check those brakes at least once a year.
A couple of more things, you will need to have a tune up (plugs, points, condensor and rotor) every year or 12k miles. New air filter every year or 12k miles. Oil change and oil filter and chassis lube every 3k or 3 months if used as a daily driver. Not used as a daily drive, then 3k or one year. Coolant change every 3 years.
Mustangs were one of the first cars to get permanently lubricated steering joints, you will need a mechanic to check if yours are that kind, if so then you won’t need a chassis lube.
